Vehicle suspensions sometimes utilize torsion bars to resiliently carry the sprung mass in a reasonably stable level attitude in spite of terrain irregularities and vehicle inertia forces. Commonly one end of each torsion bar is anchored to the road arm, and the other end is anchored to the sprung mass. The weight of the vehicle sprung mass imposes a pre-load force on the bar tending to twist the bar in wind-up fashion.
During travel over bumps the road wheel tends to move upwardly relative to the sprung mass; inertia forces in the sprung mass keep the sprung mass in a relatively stable attitude so that the bump stress is absorbed as an increased wind-up twist in the bar. During travel over holes and depressions the road wheel moves downwardly relative to the sprung mass; the bar tends to untwist or torsionally relax, while continuing to suspend the sprung mass. The torsion bar minimizes vertical excursions of the sprung mass while permitting relatively wide random up-down movements of the road wheels.
Each torsion bar of a given length has a particular torsional deflection rate, sometimes expressed as angular deflection (in degrees) per applied force (in pounds). A given length torsion bar used to suspend a relatively light vehicle will therefore undergo comparatively small total deflection as compared to the same bar when used to suspend a relatively heavy vehicle. Particularly in trucks or trailers a problem arises when the sprung mass varies widely from one trip to the next. For example, the sprung mass of an unloaded trailer may be appreciably less than one half the sprung mass of the loaded trailer, so that a given torsion bar designed to provide satisfactory wheel travel when the vehicle is unloaded will permit the vehicle to "bottom-out" when the same vehicle is loaded; conversely if the torsion bar is designed to provide satisfactory wheel travel when the vehicle is loaded then it will provide insufficient wheel travel when the vehicle is unloaded.
The present invention provides a mechanism for effectively varying the length of a torsion bar so that the same bar can provide satisfactory wheel travel whether the vehicle is loaded or unloaded. The torsion bar length is effectively varied by means of retractible anchorages located at spaced points along the torsion bar length. When the vehicle is unloaded the entire torsion bar length is utilized. Before the vehicle is loaded an auxiliary anchorage mechanism is actuated to anchor the bar at an intermediate point, thereby effectively shortening the torsion bar.